The present invention relates generally to vapor recovery canisters and in particular to on-board fuel vapor recovery canisters incorporating means for thermal management of a solid adsorbent during fueling and purging cycles so as to increase the respective adsorptive and desorptive capacities of the canister during such cycles.
As further background, during vehicle refueling operations, fuel vapor is displaced from the fuel tank by entering fuel. To prevent the displaced fuel vapors from entering the atmosphere, they are typically adsorbed in a vapor recovery canister. Such canisters are fluidly coupled to the fuel tank, and receive the fuel vapors as they are displaced. An adsorbent material contained within the canister adsorbs the fuel vapors, and a cleansed gas stream exits the canister. After the fueling operation, the adsorbent is loaded with fuel vapor, which can be desorbed as a vapor in a purge cycle and recovered for later combustion. A variety of such systems have been proposed, and for additional general background on vapor recovery systems, reference can be made for example to U.S. Pat. Nos. 5,272,873; 5,299,545; 5,235,955; 5,207,808; 5,143,041; 5,209,210; 5,156,178; 5,194,075; and 5,305,807.
The process of fuel vapor adsorption involves the condensation of the vapor on the surface of the solid adsorbent, an operation which produces heat. This compromises the efficiency of adsorption as fueling progresses because the efficiency of vapor condensation decreases with increasing adsorbent temperature. One way to address this problem might be to increase the amount of solid adsorbent in the canister. Unfortunately, this necessitates an increase in the size of the overall canister device, a result desirably avoided. Moreover, during purge operations, the passage of a gas such as ambient air through the adsorbent bed causes cooling of the bed, leading to a concomitant decrease in desorption efficiency. To address this problem, the purge gas might be heated; however, such a measure involves increased investment in both equipment and energy, and is preferably also avoided.
In light of this background there exist needs for improved vapor recovery canisters and methods which increase vapor recovery capacity without the need to increase the amount of solid adsorbent. An improved vapor recovery canister and method would desirably both increase loading capacity during a fueling operation and facilitate the removal of fuel as a vapor during a purge cycle. The present invention addresses these needs.